U.S. Pat. No. 4,968,578, Light et al issued Nov. 6, 1990; U.S. Pat. No. 4,927,727, Rimai et al, issued May 22, 1990, and U.S. Pat. No. 5,021,835, Johnson, issued Jun. 4, 1991, all describe a heat-assisted toner image transfer method. Two or more single color images are transferred in registration from an image member to a receiving sheet by heating the receiving sheet to an elevated temperature. The temperature of the receiving sheet is sufficiently above the softening point of the toner that the toner sticks to the receiving sheet. Preferably, the receiving sheet is heated from inside a transfer drum to which it is secured. The transfer drum and image member form a pressure nip with the combination of heat and pressure transferring the image. This method is particularly useful in transferring extremely small, dry toner particles, for example, toner particles having a mean particle diameter of 8 microns or less.
Especially in transferring a series of single color toner images to form a multicolor toner image, the layers of toner pile up above the level of the receiving sheet even when substantial pressure is used in transfer. This results in an unacceptable relief image corresponding generally to the optical density of the image. U.S Pat. No. 5,023,038 to Aslam et al issued Jun. 11, 1991 and U.S. patent application No. 07/405,258 to Rimai et al describe a method of fixing such toner images to a receiving sheet which receiving sheet has an outer heat-softenable thermoplastic layer. The relief image is substantially reduced, the image is more permanently fixed and gloss can be increased by bringing the image into contact with a ferrotyping surface under conditions of heat and pressure which cause the image to be further embedded in the thermoplastic layer. The ferrotyping surface is smooth and hard and has good release characteristics. For example, it can be made of nickel, stainless steel or other metals, with or without surface treating with silicones or the like. As disclosed in those references, the ferrotyping surface can also be textured to provide a matte or other textured finish to the image.
In designing a continuous production image-forming apparatus, the ferrotyping surface is formed on a web. The web is usually in the form of an endless belt, but it can also be quite long and have supply and take-up rolls for continuous operation. For purposes herein, the term "web" shall include but not be limited to an endless belt.
The use of endless belts generally to fix regular toner images to paper, transparency stock, or the like, has been known for many years; see, for example, U.S. Pat. No. 3,948,215; European Applications 0301585 and 0295901.
Japanese Kokai 1-179181; laid open Jul. 17, 1989 (Appl. No. 63-2288) shows a transfer drum for holding a receiving sheet. An internally heated fusing roller contacts the image after transfer to fix the image before it leaves the transfer drum.
U.S. Pat. No. 3,992,833, Derimiggio, issued Feb. 12, 1991 shows the use of individual intermediate sheets for receiving a toner image to which the image is fused before transfer to a receiving sheet. The intermediate and receiving sheet are maintained in contact until cool before separating.
Typically, in most of the above fixing processes the toner image is left in contact with the web until the image is cooled below the glass transition temperature of the toner, at which point the receiving sheet can be separated without offset. In the processes using a thermoplastic layer for receiving the image, that layer also is cooled to below its softening point before separation. Preventing offset by cooling in contact with the web eliminates the need for offset preventing liquids which have a degrading effect on a high quality image.
A problem in using a web system, especially an endless belt system in a productive image forming apparatus is associated with the time required for the belt and image to cool while maintained in contact. If the fixing device is slowed down to below the speed of the transfer station to allow cooling, then the mismatch of speeds between the transfer station and the fixing device must be accommodated. In general, this requires either a full frame distance in the in-track direction between the transfer station or drum and the fixing device, or a loop or other mechanism for absorbing the difference in speeds.
Belt fixing devices have other non-trivial problems associated with them. For example, belt tracking must be controlled. The belts are expensive and difficult to replace. If the belt has a seam the timing of the apparatus must be controlled to prevent the seam appearing in the middle of an image. The convenient availability of different textures is accomplished generally by exchanging belts, a task which is time consuming and especially difficult if the apparatus is hot. The belt has very limited room inside it for cooling structure.